1. Field of the Invention
This invention relates to a wet recording method in which a developing solution is used to form an image, and a wet recording apparatus for practicing the method.
2. Prior Art
In a conventional wet recording apparatus a recording electrode is held on an insulating holder, and voltage is applied between the recording electrode and a confronting electrode (cf. U.S. Pat. No. 4,330,788). In the conventional apparatus, the confronting electrode is an image bearing structure which has a conductive surface and is movable, the developing solution containing pigment particles which are moved by electrophoresis is held between the recording electrode and the image bearing structure, and the electric field formed upon application of voltage to the recording electrode acts to form an image on the image bearing structure.
The developing solution is prepared by dispersing color material particles (generally pigment particles) in a liquid dispersion medium so that, upon formation of electric fields, the particles are moved by electrophoresis. The pigment particles are treated as follows: In order to control the charging and fixing characteristics of the pigment particles, a charge control agent and pigment cover agent are applied to the surfaces of the pigment particles by adsorption or adhesion.
The above-described conventional wet recording apparatus is disadvantageous in the following points:
In the apparatus, the electric field is formed only when voltage is applied to the recording electrode, and therefore formation of the image must be accomplished while an image forming portion of the image bearing structure passes through the recording electrode. In general, the electrophoretic speed of a developing solution is low, and therefore it is necessary to increase the voltage applied to the recording electrode, or to decrease the speed of movement of the image bearing structure. However, the increase of the voltage results in an increase in manufacturing cost, and the decrease in the speed of movement of the image bearing structure results in another difficulty that the time required for obtaining a desired image is increased. In addition, since the recording electrode is held in the developing solution, the end face of the recording electrode is soiled thereby, or the pigment particles stick to the recording electrode to deteriorate the latter.
Furthermore, in the conventional apparatus, the electric field formed by application of the voltage to the recording electrodes spreads in the same manner as the electric field formed between a charged point and plane does. Hence, the image formed on a recording medium becomes larger than the width of the recording electrode, and it becomes foggy. Moreover, in the case where an image is formed directly on a conductive recording medium, it is impossible to form the image on the ordinary sheet.
One example of the conventional wet recording apparatus is an electrophotographic recording apparatus. In the apparatus, the image bearing structure is a photo-conductive drum, an electrostatic latent image on the photo-conductive drum is developed into a toner image by a developing unit, and brought into contact with a recording sheet, so that it is electrostatically transferred onto a recording sheet Electrophotographic Society Publication "Transition and Tendency of Wet Copying Machine", by Mochizuki et al, 26, 3 (1987) pp 270-276).
In the apparatus, the electrostatic latent image is converted into a toner image with high fidelity when passing through the developing unit; however, the transferred image is low in picture quality, being affected by the unevenness of the surface of the recording sheet. More specifically, if the recording sheet is low in smoothness, then the toner is transferred to the protrusions of the recording sheet, but not to the depressions, so that the toner image transferred onto the recording sheet is partially incomplete, being low in picture quality.
One example of a conventional recording apparatus for recording the same picture repeatedly is a copying machine which uses a belt-shaped photo-conductive material and dry toner. In the conventional copying machine, the belt-shaped photo-conductive material is wound on a drum one turn, and a charging operation, exposing operation, developing operation, and flash-fixing operation are carried out successively to form a block copy; and in recording an original image repeatedly, a charging operation, entire surface exposing operation, developing operation, transferring operation, and fixing operation are repeatedly carried out to form hard copies ("4th Non-impact Printing Technique Simposium Papers P113" by Yoshino, Masami).
The above-described copying machine is disadvantageous in the following points: Even when it is required to form only one image, it is necessary to wind the photo-conductive material on the drum, and the photo-conductive material used must be removed from the drum for the next image forming operation. Since these operations are carried out automatically, the copying machine is unavoidably bulky. Use of the photo-conductive material results in an increase in running cost. Furthermore, the copying machine is intricate in construction and high in manufacturing cost, because its structure requires light-shielding means.